Coverage ammonia

DEHA breaks down at high pressure. Its survival pressure is probably not in excess of 1,250 psig, but because of its high volatility and rapid reaction rate, it generally provides complete boiler cycle oxygen-control coverage. Some limited ammonia is also generated, and this may be useful for carbon dioxide neutralization. 

Deposition of TiN by the thermal decomposition of tetrakis(dimethylamido)titanium (TDMAT) in a nitrogen atmosphere (as opposed to ammonia) was characterized by a simple Arrhenius rate expression. Adequate deposition rates and good step coverage were achieved for 3/1 aspect ratio holes, 0.40 micron in size. A reactor model was designed,... 

We will list the elementary steps and decide which is rate-limiting and which are in quasi-equilibrium. For ammonia synthesis a consensus exists that the dissociation of N2 is the rate-limiting step, and we shall make this assumption here. With quasi-equilibrium steps the differential equation, together with equilibrium condition, leads to an expression for the coverage of species involved in terms of the partial pressures of reactants, equilibrium constants and the coverage of other intermediates. 

For ammonia synthesis, we still need to determine the coverages of the intermediates and the fraction of unoccupied sites. This requires a detailed knowledge of the individual equilibrium constants. Again, some of these may be accessible via experiments, while the others will have to be determined from their respective partition functions. In doing so, several partition functions will again cancel in the expressions for the coverage of intermediates. 

Figure 5.1 XPS evidence for oxygen states active in the oxidation of ammonia at Cu(110) at 290 K, for oxygen coverages of 0 = 1.0 and 0.5 and for an ammonia-rich NH3-02 mixture. Note the high activity for NH formation with the 30 1 mixture.

At Cu(l 10) surfaces, a number of different oxygen states have been investigated by STM (a) Cu(110)-O where the oxygen coverage is close to unity (b) Cu(110)-O where the oxygen coverage is < 1.0 and (c) Cu(110) exposed to an oxygen ammonia mixture. 

Simultaneously with the STM studies, Kulkarni et al,14 in Cardiff studied by XPS and HREELS the interaction of ammonia with Ni(l 10)-O and Ni(100)-0 surfaces. There was evidence in the N(ls) spectra for more than one nitrogen state present including N(a), but differentiating between NH(a) and NH2(a) was not possible. The intensity in the N(ls) spectrum region was broad over the range 397-400 eV. As the oxygen coverage increased to >0.3, the oxide O2 component became more prominent and the activity for ammonia oxidation decreased, as was observed by STM. Similar conclusions were reached for water interaction with the Ni(110)-O system.15... 

Analysis of the dynamics of SCR catalysts is also very important. It has been shown that surface heterogeneity must be considered to describe transient kinetics of NH3 adsorption-desorption and that the rate of NO conversion does not depend on the ammonia surface coverage above a critical value [79], There is probably a reservoir of adsorbed species which may migrate during the catalytic reaction to the active vanadium sites. It was also noted in these studies that ammonia desorption is a much slower process than ammonia adsorption, the rate of the latter being comparable to that of the surface reaction. In the S02 oxidation on the same catalysts, it was also noted in transient experiments [80] that the build up/depletion of sulphates at the catalyst surface is rate controlling in S02 oxidation. 

Figure 3 Variations with coverage of the differential heats of adsorption of ammonia on H-ZSM-5 (sample 1) measu-red at 150°C, (A), 200°C, (a), 250°C, (fe 300°C (0) and 400°C (+) The sample was outgassed at 400°C prior NH3 adsorp-tion. The meaning of the arrows is explained in the text.

Figure 1.10 Differential heats of adsorption as a function of coverage for ammonia on H-ZSM-5 (o) and H-mordenite ( ) zeolites [70], In both cases, the heats decrease with the extent of NH3 uptake, indicating that the strengths of the individual acidic sites on each catalyst are not uniform. On the other hand, the H-ZSM-5 sample has a smaller total number of acidic sites. Also, the H-mordenite sample has a few very strong sites, as manifested by the high initial adsorption heat at low ammonia coverage. These data point to a significant difference in acidity between the two zeolites. That may account for their different catalytic performance. (Reproduced with permission from Elsevier.)...

The constant ratio found spectroscopically is, perhaps, unexpected. The largest coverage, however, for ammonia in the spectroscopic experiments, was 0.18 at 150° C. and 0.1 for acetone at 75° C. Under these conditions,... 

Figure 13.5 Adsorption-desorption of ammonia at 280 " C on a model V2O5—WO3/TiO2 catalyst Dashed lines, inlet NH3 concentration triangles, outlet NH3 concentration solid lines, model fit with Temkin-type coverage dependence. Adapted from ref. [3].

This empirical rate expression considers the active sites of the catalyst as only a fraction of the total adsorption sites for ammonia and is consistent vfith the presence of a reservoir of ammonia adsorbed species which can take part in the reaction. The ammonia reservoir is likely associated vfith poorly active but abundant W and Ti surface sites, which can strongly adsorb ammonia in fact, nhs roughly corresponds to the surface coverage of V. Once the ammonia gas-phase concentration is decreased, the desorption of ammonia species originally adsorbed at the W and Ti sites can occur followed by fast readsorption. When readsorption occurs at the reactive V sites, ammonia takes part in the reaction. Also, the analysis of the rate parameter estimates indicates that at steady state the rate of ammonia adsorption is comparable to the rate of its surface reaction with NO, whereas NH3 desorption is much slower. Accordingly, the assumption of equilibrated ammonia adsorption, which is customarily assumed in steady-state kinetics, may be incorrect, as also suggested by other authors [55]. 

The amphoteric indium oxide can be considered as more basic than acidic when comparing the adsorption heats and irreversible adsorbed amounts, which are clearly higher for SO2 adsorption than for ammonia adsorption [40,47]. The heats of NH3 adsorption decreased continuously with coverage, while the SO2 adsorption heat remained constant over a wide range of coverage. 

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